make compatible with ForwardDiff at lower bound of extrapolation object (#336)

Author: taylormcd

src/extrapolation/extrapolation.jl

@@ -123,7 +123,7 @@ function inbounds_index((flagl,flagu)::Tuple{Nothing,Throw}, (l,u), x, etp, xN)
 end
 
 function inbounds_index((flagl,flagu)::Tuple{Union{Flat,Line},Flag}, (l,u), x, etp, xN)
-    inbounds_index((nothing,flagu), (l,u), maxp(x,l), etp, xN)
+    inbounds_index((nothing,flagu), (l,u), maxp(l,x), etp, xN)
 end
 function inbounds_index((flagl,flagu)::Tuple{Nothing,Union{Flat,Line}}, (l,u), x, etp, xN)
     minp(x,u)

test/issues/runtests.jl

@@ -129,4 +129,21 @@ using Interpolations, Test, ForwardDiff
         j = ForwardDiff.gradient(itp_test, [2.0])
         @test j ≈ Interpolations.gradient(build_itp(), 2.0)
     end
+
+    @testset "issue 335" begin
+        function build_itp()
+            A_x = 1:5
+            A = A_x
+            knots = (A_x,)
+            LinearInterpolation(knots, A, extrapolation_bc=Line())
+        end
+        function itp_test(x::AbstractVector)
+            itp = build_itp()
+            return itp(x...)
+        end
+
+        @test ForwardDiff.gradient(itp_test, [1.0]) ≈ Interpolations.gradient(build_itp(), 1.0)
+        @test ForwardDiff.gradient(itp_test, [5.0]) ≈ Interpolations.gradient(build_itp(), 5.0)
+    end
+
 end